Integrated circuit (IC) dies are encapsulated in protective packages to allow easy handling and assembly onto printed circuit boards (PCBs) and to protect the dies or related components and devices from damage. A dielectric material, such as a hard plastic, is typically used to encapsulate the dies and related components to form the package. A variety of different IC package types exist, and several types include heat spreaders formed on one or more sides (typically the top side) of the package. Specifically, heat spreaders are interconnected to the encapsulated dies, and possibly other encapsulated components, to function as a heat sink and thereby assist in drawing heat out of the package.
As might be expected, the technique for physically (and thus thermo-conductively) connecting the heat spreader to the encapsulated die(s) and other components is crucial in ensuring proper heat dissipation. Cracks or other physical failures between the underlying dies and the heat spreader can result in insufficient heat dissipation, which may result in die overheating and in turn, die failure. This is usually caused due to a mismatch in the coefficient of thermal expansion (CTE) of the silicon typically used to form the underlying dies (a CTE of 3 ppm/K) and of copper typically used to form the heat spreader (a CTE of 16 ppm/K). Thus, multiple techniques and structures for physically connecting the encapsulated dies with a heat spreader exist; however, even the more popular conventional approaches can suffer catastrophic failure.
One conventional approach is to directly contact the top (e.g., via a thermal pad) of an encapsulated die with the heat spreader. Looking at FIG. 1, illustrated is a cross-sectional view of a block diagram of a conventional IC package 100 providing a direct connection between a die and a heat spreader. Specifically, the package 100 is comprised of a circuit layer 105 and a redistribution layer (RDL) 110. The circuit layer 105 includes the IC dies 115, as well as other components such as a drive IC 120. The RDL 110 typically includes a number of conductive vias 122, such as metal pillars, and interconnects 124, such as a metal leadframe, to provide electrical connection to electrically conductive bond pads on the die 115 and other components to other underlying circuitry.
The package 100 also includes a heat spreader 125, which is typically composed of copper due to its excellent thermal-conductivity. In order to dissipate the heat from the die 115 to the heat spreader 125, an interface 130 provided by the direct physical connection between the two exists. In some embodiments, a plain metal layer, which is typically also copper, may be provided as the direct interface 130 of the die 115 and the heat spreader 125. However, due to the large mismatch in CTE at the direct interface 130 between the silicon of the die 115 and the copper of the heat spreader 125 (or metal layer), cracks typically develop at their interface 130 when the package 100 is subjected to high temperatures. For example, embedded packages are typically heated to a temperature above the solder melting temperature, typically during an infrared (IR) reflow process, to remove moisture the plastic encapsulant may have absorbed during the manufacturing process. During such an IR reflow process, the CTE mismatch between the heat spreader 125 and the top surface of the die 115 can result in such cracking, with this loss in structural integrity often significantly affecting the heat dissipation capabilities of the heat spreader. Such cracking may also occur in response to the high temperatures experienced in extreme operating conditions for the package 100.
Turning to FIG. 2, illustrated is an image from a scanning electron microscope (SEM) of a used conventional package 200 employing a direct physical connection between an encapsulated die and a heat spreader. As with the diagram in FIG. 1, the package 200 in FIG. 2 also includes a circuit layer 205 as well as an RDL 210. An IC die 215 can be seen encapsulated within a dielectric material in the circuit layer 205. The scan also shows a copper heat spreader 225 directly connected to the top of the die 215 for heat dissipation. Over time, a large crack 230 can be seen formed at the interface between the die 215 and the heat spreader 225 caused by the mismatch in thermal expansion between these two components of the package 200. FIG. 2A provides a close up view of a portion of the package 200 in FIG. 2. In this close up view, the crack 230 proximate to the interface between the die 215 and the heat spreader 225 can be easily be seen. Consequently, the ability of the heat spreader 225 of this conventional package 200 to dissipate heat from the die 215 has been compromised.
Looking now at FIG. 3, illustrated is a cross-sectional view of a block diagram of another conventional IC package 300 providing an indirect connection between a die and a heat spreader. This conventional package 300 again includes a circuit layer 305 and an RDL 310. The circuit layer 305 includes an IC die 315 in need of heat dissipation using a heat spreader 325. In this conventional approach, thermally conductive vias 335 are used to provide a physical, and thus thermal, connection between the die 315 and the heat spreader 325. To provide good thermal conductivity, the vias 335 are also typically formed of copper along with the heat spreader 325, again due to copper's good thermal conductivity. In addition, a thin conductive seed layer 340, usually copper as well, may also be formed on top of the die 315 to assist with thermal conductance across the top of the die 315, as well as provide contact with the bottoms of the conductive vias 335.
Unfortunately, as with the prior approach, this second conventional approach also suffers from structural disadvantages. In particular, although the conductive vias 335 and the seed layer 340 are both typically formed of copper, the interfaces where the vias 335 contact the seed layer 340 still suffer from mechanical failure. Specifically, the interface between the bottoms of each conductive via 335 and the top of the die 315 typically suffer structural failure, again usually in the form of cracks, not due necessarily to CTE mismatch, but instead due to high thermo-mechanical stress resulting from the relatively small diameter for each of the bottoms of the conductive vias 335 even though connected to the similar composition seed layer 340. This typically again occurs when the package 300 undergoes through an IR reflow process. This loss in structural integrity can again significantly affecting the heat dissipation capabilities of the heat spreader 375.
Turning briefly to FIG. 3A, provided is an SEM image 350 of a used conventional package employing conductive vias between an encapsulated die and a heat spreader. The image 350 shows this embodiment of a conventional package also includes a circuit layer 355 and RDL 360, with a silicon IC die 365 encapsulated in the circuit layer 355. The heat spreader 375 can be seen located above and spaced from the die 365, with copper vias 385 providing physical, and thus thermal, connections between the die 365 and the heat spreader 375. In addition, a thin conductive seed layer 390 can also be seen deposited over the top of the die 365 to assist with heat dissipation, with the copper vias 385 being physically connected to the top of the seed layer 390 at interfaces 380.
However, as mentioned above, structural failure in the form of cracks still typically form at the interfaces 380 between the vias 385 and seed layer 390 after the package undergoes an IR reflow process or other cause of similar high temperatures. This is caused by the downwardly tapered sidewalls of the vias 385 that result from the formation process employed to form the conductive vias 385. Specifically, once dielectric material is deposited on and around the circuit layer 355 to encapsulate the die 365 and other components and interconnects in the circuit layer 355, the encapsulation material is deposited to a predetermined height about the die 365. Then, laser drilling is used to form the vias 385 by drilling down through the encapsulation material to reach the top of the die 365 or the top of the seed layer 390. Typical metal deposition techniques may then be used to fill the drilled holes with copper to form the vias 385. Thereafter, copper deposition may again be used to form the heat spreader 325 on top of the encapsulation material, and in contact with the tops of the copper vias 385.
Unfortunately, the laser drilling process create via openings that are tapered from their tops to their bottoms. As a result, the filled conductive vias 385 have a corresponding downward, cylindrical tapering. This tapering of the vias 385 results in a via structure with a narrow diameter at the interface 380 with the die 365 or seed layer 390. These conventional tapered via 385 structures result in less overall copper surface area conducting heat from the seed layer 390 or die 365, which results is less heat dissipation. In addition, this relatively narrow interface 380 results in a high thermo-mechanical stress at the interface 380 of each via 385, for example, after the package undergoes a reflow process. Consequently, similarly to the direct conductive connection of the other conventional approach discussed above, the high thermo-mechanical stress at the interfaces 380 will result in cracks being formed their interfaces 380 of the bottom of each via 385, especially in extreme operating conditions for the package 300. Also as before, such cracks result in mechanical failure for the heat dissipation intended to be provided by the conductive vias 385. Furthermore, the inwardly tapered structure of the bottom portions of the vias 385 provides less overall thermal conductive surface contacting the die 365 or a seed layer 390 over the top surface of the die 365
Accordingly, what is needed in the art is a dissipation structure for encapsulated packages, and related methods for manufacturing such dissipation structures, that do not suffer from the deficiencies of the prior art. The disclosed principles provides these and other improvements.